Eocene Volcanism Research Articles

The role of clinopyroxenes in the process of formation of subalkaline olivine-basaltic magma of the Talysh zone (Azerbaijan)

Based on the study of the compositions, relationships of coexisting minerals, as well as the minal components of clinopyroxenes of trachybasalt-trachyandesibasalt-phono- lite formation of the Talysh zone, it was established that they characterize different stages of crystallization of subalkali In addition, the role of clinopyroxenes in the process of formation of subalkaline olivine-basaltic magma of the Talysh zone was determined. The characteristics of the Eocene volcanism of trachyba- salt-trachyandesite-basalt-phonolite formation of the Talysh zone were studied. Using new analytical data of author’s materials, the composition of rocks and minerals of the trachybasalt-trachyandesite-basalt-phonolite formation of the Talysh zone were analyzed. Analyses of rocks and parts of clinopyroxenes were carried out in the chemical laboratory “Uralgeology” of the Ministry of Geology of the Russian Federation (Ekaterinburg). The composition of clinopyroxenes and the melt inclusions they contain were determined by microprobe analysis (Camebax-micro) in the corresponding laboratories of the United Institute of Mineralogy of the Russian Academy of Sciences (Novosibirsk). It was determined that in the complex, under the conditions of the Astara uplift, the rise of subalkaline olivine-basaltic melt into the upper horizons of the earth’s crust was solidified and the early accumulative cumulates of rock minerals in intermediate sources had time to completely dissolve. In the Gosmolyan-Pilachai trough, with the mobility of its transverse mag- ma-supplying faults, such a melt did not have time to crystallize and fractionate. As a result, the transition between petrographic types of rocks in this case is gradual and accumulative inclusions of rocks, minerals were preserved in the melt. The first case is noted by crystallized melt microinclusions, represented by biotite, potassium feldspar, residual glass, the composition belonging to trachydacite and trachyrhyolite, their homogeneous temperature genesis fluctuates between 900-1000oC. In the second case, crystallites of leucite, phlogopite, olivine, etc. were found and molten microinclusions in composition are close to the main alkaline varieties of rocks of the complex under consideration.

The origin of the Musicians Seamount Province and its inferences for Late Cretaceous Pacific Plate Motion

Linear chains of seamounts, sourced from mantle plume processes, have the potential to refine plate motion models because the hotspot remains fixed relative to the moving lithospheric plate. However, to define plate motion, consistent seamount age progression and geometry are required. Some seamount chains, such as the Musician Seamount Province (MSP), have complex geometries and age distributions, which complicates calibrating plate motion. The MSP resides northwest of the Hawaiian Islands and is composed of seamounts and volcanic elongated ridges (VERs) that cover ∼420,000 km2 of Pacific seafloor. Here we provide new 40Ar/39Ar age determinations for a series of lava flows recovered from the MSP during expedition EX1708 of the National Oceanic and Atmospheric Administration's Ocean Exploration program. The MSP was built by four distinct volcanic processes: (1) age-progressive hotspot volcanism associated with the Euterpe Plume (ca. 98–79 Ma). (2) VER formation from plume-ridge channelization (ca. 97–94 Ma; 86–79 Ma) where the VERs only form when the hotspot is within ∼600 km of the ridge. (3) Eocene volcanism driven by extension during the ca. 50 Ma change in Pacific rotation poles (ca. 54–47 Ma). (4) Some near-ridge shear-driven upwelling or diffuse extensional volcanism that preceded the southern MSP lithosphere overriding the plume (ca. 86–84 Ma). By filtering lava flows with only robust statistically concordant 40Ar/39Ar age determinations as well as geologic setting, we develop a dataset of samples valuable for constraining Pacific plate motion. A local plate velocity of 42 ± 9 km/Ma for the 98–81 Ma time frame is calculated. Furthermore, the seamount track indicates that large shifts in Pacific rotation pole locations are required prior to 98 Ma and at ca. 81 Ma.

Geodynamic Evolution of the East African Superplume: Insights From Volcanism in the Western Turkana Basin

AbstractThere is a consensus that volcanism along the East African Rift System (EARS) is related to plume activities. However, because of our limited knowledge of the local lithospheric mantle, the dynamics of the plume are poorly constrained by magma chemistry. The Turkana Basin is one of the best places to study plume‐related volcanism because the lithospheric mantle there is unusually thin. New Ar‐Ar geochronology and geochemical data on lavas from western Turkana show that Eocene volcanics have relatively low 206Pb/204Pb (<19.1) and high εNd (>3.78). Their relatively high Ba/Rb (35–78) ratios suggest contributions from the shallow lithospheric mantle. Oligo‐Miocene Turkana volcanics have HIMU‐ and EMI‐ type enriched mantle signatures with overall lower Ba/Rb ratios, which is consistent with partial melting of plume material. Pliocene and younger Turkana volcanics have low Ba/Rb and Sr‐Nd‐Pb isotope ratios that resemble those of Ethiopian volcanics with elevated 3He/4He ratios. This temporal variation can be reconciled with a layered plume model where an outer layer of ancient recycled oceanic crust and sediment overlies more primitive lower mantle material. Beneath Ethiopia, the outer layer of the plume is either missing or punctured by the delamination of the thicker overlying lithospheric mantle at ca. 30 Ma, an event that would have facilitated the rapid upwelling of the inner portion of the plume and triggered the Ethiopian flood volcanism. The outer layer of the plume may be thicker in the southern EARS, which could explain the occurrence of young HIMU‐ and EMI‐type volcanics with primordial noble gas signatures.

Petrography and geochemistry of the Miocene Upper Red Formation sandstones in NW Iran; with an application to the origin and tectonic setting

Middle to Upper Miocene sedimentary successions in the Alborz sedimentary basin (northern Iran) form the caprock of the Lower Miocene gas reservoirs. Their stratigraphic and sedimentological studies have been of interest to geological studies. Due to their importance, in this research, two suitable stratigraphic sections in the Zanjan and Hashtroud regions (northwest of Iran) have been subjected to detailed sedimentological and stratigraphic study to determine the origin and tectonic setting. Clastic-evaporite sediments of the middle to upper Miocene (Upper Red Formation) in the northwest of Iran are about 950 m thick, which is placed on the hydrocarbon reservoirs of the Qom Formation (Lower Miocene) with a discontinuous and abrupt boundary, and are covered by conglomerate and unhardened sediments belong to Pliocene. These sediments in the northwest of Iran are divided into four lithostratigraphic units, including (1) green marls with gypsum, (2) red marls along with gray and red sandstones, (3) red and green marls, and (4) green marls. Based on microscopic studies, the sandstones of this formation are composed of sedimentary and metamorphic rock fragments, quartz, and to a lesser extent, feldspar, pyrite and chlorite, in order of importance. The matrix (carbonate mud) and, to a lesser extent, calcite and evaporite cements form the groundmass of studied sandstones. Based on petrographic studies, these fine-to medium-grained sandstones are litharenite to feldspathic litharenite with an average composition of Q38F13RF49. Also, based on the main constituent components of sandstones and geochemical analysis of major and minor elements, and by drawing different types of separating diagrams, the tectonic setting of these sandstones are considered as an active continental margins and foreland basins (collision belts between Arabia-Eurasia continental plates), which originate from fine-to medium-grained siliceous-clastic rocks, metamorphic, carbonate, and intermediate to felsic igneous rocks outcropped in the region. The data show that these sediments are influenced by arid to semi-arid climate and moderate degree of chemical weathering, mainly from the outcrops of Takab Complex and Qom Formation (Lower Miocene), which have significant outcrops and, to a lesser extent, from Eocene volcanics. The research results are consistent with the paleogeography of the region and previous studies and can be of interest to sedimentologists and other researchers related to petroleum geology.

Interpretation of Geochemical Data of Eocene Volcanism in Eastern Sivas Province (Central Anatolia, Türkiye)

The study area covers the eastern part of Zara district (Sivas) located in East-Central Anatolia. The units found in the region from bottom to top are characterized by the Upper Creataceous-Paleocene Refahiye Complex, the Middle-Upper Eocene Akıncılar Formation, the Karataş volcanics and Kösedağ Syenite, the Lower Miocene Onarı Formation, the Upper Miocene-Pliocene İsola volcanics, the Şerefiye volcanics, the Kadıköy Formation and the uppermost Quaternary sediments. In the frame of this study, the Karataş volcanics, which were formed as a result of the second magmatic activity of the Eocene period, are studied in detail. The Karataş volcanics are greenish black on their undisturbed clean surfaces, and some parts are brown-purple in color and have distinctive crack systems. It shows a reddish-pinkish color distribution in regions where degradation is observed intensely. The SiO2 content, which is the main criterion for determining the magma character of Karataş volcanics, is between 52.09 % and 58.45 %. MgO contents of similar importance are between 0.53 % and 2.21 %. Some of the light rare earth elements, La have 22.2-40.7 ppm, Ce 44.4-67.3 ppm, Pr 5.23-8.88 ppm, Sm 4.41-7.15 ppm, Eu 1.24-1.83 ppm; one of the heavy rare earth elements, Lu have 0.20-0.45 ppm and Yb 1.88-2.98 ppm values. The Karataş volcanics consist of basaltic-trachyandesite and trachyandesite. Evidence of liquid-crystal fractional crystallization (FC) in the phenocryst phase during the formation of the Karataş volcanics is clearly observed in the geochemical diagrams. Traces of partial melting and crustal contamination were found in geochemical studies. Following the closure of the Neo-Tethys ocean in the region, a widespread crustal thickening occurred in the region with the northward divergence of the northern branch of Neo-Tethys, and a suture zone was formed along this line. The Karataş volcanics were formed as a product of active volcanism after the collision in the region.

Determination of alteration zones applying fractal modeling and Spectral Feature Fitting (SFF) method in Saryazd porphyry copper system, central Iran

The target of this research is recognition of the alteration zones utilizing concentration-area fractal methodology according to the reflection of the main minerals of each alteration zone that enhanced by Spectral Feature Fitting (SFF) method due to Advanced Space-borne Thermal Emission and Reflection Radiometer (ASTER) satellite images in Saryazd porphyry system, central Iran. The alterations and mineralization developed in Eocene volcanics. Remote sensing results achieved by the SFF method and Concentration -Area (C - A) fractal modeling represent different parts of propylitic, argillic and phyllic alteration zones due to their intensity and pixel values. In addition, the results reveal that there is a ring-shaped structure in the alteration zones, which are correlated with results, derived from X-Ray Diffraction (XRD) analyses and field observations.

Some geological and petrological characteristics of the Eocene trachybasalt-trachyandesite- basalt-phonolite formation of the Talysh zone (Azerbaijan)


 
 
 This article discusses the stages of crystallization of the Eocene trachybasalt-tra- chyandesitebasalt-phonolith formation of the Talysh zone. Determination of the composi- tion of the primary magma, evolution and melting fraction of the initial melt of the Eocene volcanism of the Talysh zone within Azerbaijan. Eocene volcanism of the trachybasalt-tra-chyandesitebasalt-phonolith formation of the Talysh zone. Petrographic, petrochemical, geochemical studies of rocks, typomorphic features of mineral paragenesis, chemical composition of rocks were carried out by X-ray spectral and X-ray fluorescence analysis. The analyzes of the conducted studies show that the Talysh zone was formed in the Eocene time. Stages of evolution of the original magma and typomorphic features of mineral parageneses have been established. The distributions of microelements in mineral parageneses, which are direct participants in the crystallization processes that took place in intermediate chambers at different depths, have been studied. At present, the question arises of the need to clarify the typomorphic features of igneous complexes, as factors that determine the geodynamic regimes of their formation, and to determine the potential ore content. The petrographic and petrochemical characteristics of the volcanic rocks are given. It was found that if the process of differentiation in the Early-Middle Eocene was more distinct with the initial formation of more magnesian rock varieties (picrite-trachybasalts), and subsequently more ferruginous ones (trachyandesites and trachyandesitebasalts), then such accumulation of iron is not observed in porphyritic trachyandesites. This petrochemical feature is obviously associated with the duration of the break in volcanism, caused by the formation of a thick sedimentary-tuffaceous stratum, accompanied by the initiation and manifestation of an intermediate chamber of medium composition. Volcanism in the Late Eocene was undifferentiated and was not accompanied by the formation of medium differences. The appearance in the late Eocene of leucite phonolites, which represent an alkaline branch, and the absence of transitional varieties indicate the autonomous development of vit- robasalts and leucite phonolites. It is concluded that from the early phases of the manifestation of Eocene volcanism to the later phases, the change in the material composition along the lateral is expressed in the manifestation of more alkaline facies with a significant predominance of K over Na and with a greater correspondence to the differentials of the shoshonite series.
 
 

Eocene Volcanic Complex from Central British Columbia: The Role of Fractional Crystallization during the Magmatic Evolution

Abstract The Challis-Kamloops belt of south-central British Columbia is a regionally extensive (>65,000 km2) magmatic province that erupted within the North American Cordillera during the Eocene (55-45 Ma). The inland volcanic belt runs parallel to the coast, and the rocks were emplaced mainly within extensional basins indicating volcanism was attributed to rift-related decompressional melting. The rocks include both calc-alkaline and tholeiitic mafic and intermediate types (i.e., low-Fe, medium-Fe, and high-Fe suites). Voluminous volcanic units (Buck Creek, Goosly Lake, Swans Lake) of the Buck Creek volcanic complex (~3,000 km2 in area) within the Nechako plateau erupted within 1-2 million years and show significant internal chemical variability. All rock types have similar Sr-Nd isotopic (87Sr/86Sri=0.70435-0.70487; εNdt=+2.6-+4.0) ratios indicating they originated from the same sub-Cordilleran mantle source. Petrological modeling using the most primitive rocks of the Buck Creek, Goosly Lake, and Swans Lake magmatic pulses demonstrates that the chemical variability observed in each system can be explained by hydrous fractional crystallization in the upper crust (≤0.1 GPa) under moderately oxidizing to oxidizing conditions (ΔFMQ 0 to +0.7). The primary difference between the low-Fe to medium-Fe (calc-alkaline) Buck Creek suite model and the high-Fe to medium-Fe (tholeiitic) Swans Lake suite model is water content as the Swans Lake model has lower (H2O=0.75 wt.%) starting water than the Buck Creek and also the Goosly Lake models (H2O=1.25-2.00 wt.%). Moreover, the intermediate to silicic rocks of the complexes are compositionally similar to rocks associated with “slab failure” suggesting that rifting and mantle melting were related to asthenospheric upwelling through a slab tear. The implications are that the chemical variability of the rock suites are primarily related to fractional crystallization and that the mantle source is heterogeneous with respect to water content which is likely due to heterogeneities in the processes related to pre-Eocene subduction.

Petrogenesis of post-collisional Middle Eocene volcanism in the Eastern Pontides (NE, Turkey): Insights from geochemistry, whole-rock Sr-Nd-Pb isotopes, zircon U-Pb and 40Ar-39Ar geochronology

The debate about whether Eocene magmatism is considered to be post-collisional or subduction-related or not still continues. Here we offer new 40Ar-39Ar ad U-Pb zircon geochronology, mineral chemistry, bulk rock and Sr-Nd-Pb isotope geochemistry data obtained from the southern dike (SD) suite, in comparison with the northern dike (ND) suite, from the Eastern Pontides. The geochronological data indicate that the SD suite erupted between 45.89 and 45.10 Ma corresponding to the Lutetian (Middle Eocene). The magmas of the ND suite are characterised by slightly more alkaline affinity compared to the SD suite. The trace and rare earth element (REEs) content of the SD suite is characterised by large ion lithophile element (LILEs; Sr, K2O, Ba, Rb) enrichment and depletion of Nb, Ta, and TiO2 elements to different degree with high Th/Yb ratios, which indicate that the magmas forming the SD and ND suites were derived from lithospheric mantle sources enriched by mostly slab-derived fluids in the spinel stability field. The Sr, Nd and Pb radiogenic isotope ratios of the dikes support the view that the magma for the hydrous group (H-SD) was derived from a relatively more enriched mantle source than the other SD and ND suites. The ND suite and the anhydrous group (A-SD) display similar geochemical features characterised by moderate light earth element (LREE)/heavy rare earth element (HREE) ratios, while the H-SD group has respectively lower LREE/HREE ratios indicating higher melting degree. Detailed considerations of the alkalinity, enrichment and partial melting degree for the source of the studied volcanic rocks indicate that the magmas of the northern dike suite are characterised by slightly more alkaline affinity, whereas the magmas throughout the southern dike suite show increments in the enrichment rate and melting degree. In light of the obtained data and comparative interpretations, the geodynamic evolution and differences in petrogenetic character of the Lutetian magmas from both the northern and southern parts of the Eastern Pontides may be explained by different degrees of melting of a net veined mantle source initially metasomatized by mostly subduction fluids during asthenospheric upwelling due to fragmented asymmetric delamination in a post-collisional extensional tectonic environment.

TECTONOMAGMATIC EVOLUTION OF THE EOCENE VOLCANISM OF THE TALYSH ZONE (AZERBAIJAN)

Link for citation: Karimov V.M. Tectonomagmatic evolution of the Eocene volcanism of the Talysh zone (Azerbaijan). Bulletin of the Tomsk Polytechnic University. Geo Аssets Engineering, 2021, vol. 332, no. 11, рр. 200-211. In Rus. The relevance. At present, the question arises of the need to clarify the typomorphic features of magmatic complexes, as factors that determine the geodynamic regimes of their formation and to clarify the potential ore content. In this regard, the article examines the features of the Talysh magmatic complexes, which makes it possible to elucidate the tectonomagmatic evolution of this complexly constructed region in the Caucasus. Purpose of research is to clarify the tectonomagmatic evolution of the Eocene volcanism of the Talysh zone within Azerbaijan. Objects: Eocene volcanism, volcanogenic trachybasalt-trachyandesite-basalt-phonolite formation, petrochemical composition of rocks, evolution of the primary melt. Methods: petrographic studies of rocks, chemical composition of rocks by X-ray spectral analysis, isolation of magmatic complexes. In addition, the results of analyzes of the studied samples for petrogenic elements were used, both published in the literature and obtained by X-ray fluorescence analysis on a multichannel X-ray spectrometer SRM-25 at the Institute of Geology and Geophysics, Baku. Minerals were analyzed on a JXA-8200 JEOL (Japan) electron microprobe at the Karpinsky All-Russian Geo-logical Research Institute (St. Petersburg). Results. The analysis of the conducted studies shows that during the period from the Paleozoic to the Quaternary, inclusive, in the development of the structures of the Talysh fold zone, there is a certain sequence in its inception. In the structure of the zone, there are structural stages corresponding to the pre-collisional (Cretaceous–Eocene) and collisional (Oligocene–Miocene) periods of the region's development. Paleogene volcanic formations of Talysh make up the southeastern segment of the northern volcanoplutonic belt, in the structure of which the volcanic trachybasalt-basaltic trachyandesite-phonolite (Early-Middle and Late Eocene) and intrusive subalkaline ultrabasic (Late Eocene–Oligocene) formations are distinguished. The volcanic formation includes two complexes: (Early-Middle Eocene) absarokite-shoshonite-alkaline basaltic and (Late Eocene) basaltic trachyandesite-phonolite. The petrographic and petrochemical characteristics of the volcanic rocks that make up the first formation are given. It was found that if the process of differentiation in the Early-Middle Eocene had more distinct character with the initial formation of more magnesian rock varieties (picrite-trachybasalts), and subsequently more ferruginous (trachyandesites and basaltic trachyandesites), then such accumulation of iron is not observed in porphyry trachyandesites. This petrochemical feature is obviously associated with the duration of the break in volcanism caused by the formation of a thick sedimentary-tuffaceous sequence, accompanied by the formation and manifestation of an intermediate chamber of average composition. Volcanism in the Late Eocene was undifferentiated and had a contrasting character, i. e. was not accompanied by the formation of average differences. The appearance in the Late Eocene of leucite phonolites, which are an alkaline branch, and the absence of transitional differences indicate the autonomous development of vitrobasalts and leucite phonolites, possibly associated with the process of early decomposition of the initial melt under subcrustal conditions. It is concluded that from the early phases of the manifestation of Eocene volcanism to the later, the change in the material composition laterally is expressed in the manifestation of more alkaline facies with a significant predominance of K over Na and with a greater correspondence to the differentiates of the Shoshonite series.

Eocene volcanism in the Fuegian Andes: Evidence from petrography and detrital zircons in marine volcaniclastic sandstones

The sedimentology, petrography, and U–Pb dating of two Eocene volcaniclastic horizons of the Punta Torcida and Leticia formations, Austral basin, Tierra del Fuego, Argentina are interpreted and documented. The volcaniclastic deposits, pumicite breccia and tuffaceous sandstones, are formed by glass shards, plagioclase crystals, and pumiceous and lithic andesitic fragments. Originally deposited as tephra fallout, they have been subsequently reworked and redeposited in marine settings. The final deposits, however, are interpreted essentially as syn-eruptive. U–Pb dating of detrital zircons gave a 46.3 ± 0.4 Ma (early Lutetian) age for the top of the Punta Torcida Formation and 41.9 ± 0.71 (late Lutetian) to 39.6 ± 0.82 (Bartonian) ages for the Leticia Formation. Paleogene volcanic rocks are unknown in the Southern Patagonian-Fuegian Andes; hence the studied volcanic deposits with minimal reworking are important to evaluate the time lag between eruption and true depositional ages in detrital zircons. The resulting dates allow evaluating the timing of the important, basin-wide, intra-Eocene unconformity known from Tierra del Fuego to Lago Argentino, Santa Cruz, Argentina. The documented eruptive phases are related to Andean magmatism, probably located in the outcrop area of the Seno Año Nuevo suite in the Chilean Archipelago.

Petrology of Eocene Volcanic Rocks in Dastgerdo to Molla Ahmad Area (East of Isfahan); Middle Part of the Urumieh- Dokhtar Magmatic Arc (UDMA)

Petrology of Eocene Volcanic Rocks in Dastgerdo to Molla Ahmad Area (East of Isfahan); Middle Part of the Urumieh- Dokhtar Magmatic Arc (UDMA)

İZMİR-ANKARA-ERZİNCAN SUTUR ZONUNDAKİ (SİVAS KUZEYDOĞUSU) EOSEN YAŞLI KARATAŞ VOLKANİTLERİNİN JEOKİMYASAL ÖZELLİKLERİ

Karatas volcanics, is the product of Eocene volcanics crop out in the form of two belts along the northern and southern boundaries of the Izmir-Ankara-Erzincan Suture Zone. According to geochemical data, these volcanics have alkaline basic-intermediate character and consist of basaltic trachyandesite, trachyandesite and trachyte. This volcanic activity has been controlled by fractional crystallization and crustal contamination from basaltic trachyandesite to trachyte. Orientation of the samples towards amphibole area on the Rb/Sr - Ba/Rb ratio diagrams, dispersion of the Zr/Ba ratios (0.08 - 0.33) in the lithospheric mantle range, increase in the Ba/Rb ratio, decreases in the MgO, Ni and Cr contents point out that this volcanism originated from enriched lithospheric mantle rather than asthenospheric mantle. Geochemical data show that this enriched lithospheric mantle material is upper continental crustal material, main part of enrichment resulted by the subduction related fluids and also the contribution of the sedimentary materials. This situation may be explained that; the melts, derived from N-MORB or OIB bearing material ascended into the continental crust in a pre-collisional period and were reactivated by extensional tectonic and/or delamination processes during the post-collisional period, possibly caused the partial melting within the upper continental crust and produced the Karatas volcanics.

Petrology and Geochemistry of the Eocene Volcanic Rocks in the West of Sechangi, Lut Block

Petrology and Geochemistry of the Eocene Volcanic Rocks in the West of Sechangi, Lut Block

Compressional evolution of the PNG margin from an orogenic transect from Juha to the Sepik

A crustal-scale, fully restored section across the PNG orogenic belt reveals the Oligocene to Recent compressional deformation of the margin. The northern end of the section comprises the Landslip Metamorphics, an accreted continental terrane, separated from the main part of the Fold Belt by the Jurassic April Ultramafics and Om Metamorphics, interleaved with Eocene volcanics which together constitute an accretionary prism. Existing maps show that the suture is overlain by distal Miocene sediments indicating Oligocene docking and probable compression prior to Early Miocene subsidence. The latter is consistent with Early Miocene extension in PNG and the emplacement of metamorphic core complexes in the Sepik area, but is also related to dynamic topography causing subsidence of the whole northern margin of Australia. Neogene compression commenced around 12 Ma with ~70km shortening in the Om terrane and ~38km shortening in the Fold Belt. Existing thermochronology data indicate shortening of ~12mm/year from 12-4 Ma, but only 2.5mm/year from 4-0 Ma, consistent with a change in structural style in the Fold Belt from thrust to more ductile, fold-dominated deformation. The model also requires substantial thickening of the continental crust beneath the Muller Ranges, here represented by ‘basement’ underthrusting. Gravity modelling indicates the presence of sedimentary graben up to 10 km deep beneath the Fold Belt, which were strongly inverted, such as beneath the Lavani Valley. A key issue is when this inversion occurred, in the Oligocene or Pliocene, as this has a significant influence on the timing of hydrocarbon generation and migration.

A sedimentary model of mountainous rivers with application to Alamutrud and Shahrud rivers, N Qazvin, Iran

The most important depositional processes in continental sedimentary basins occur in fluvial systems. In this study, sedimentology of Shahrud River and its eastern tributary (Alamutrud River, N Iran) was studied for the first time. The study area is located in the Southern-Central Alborz structural zone which is mostly composed of Eocene volcanics and Miocene terrigenous sedimentary rocks. The channel trend is controlled by the regional structural trends. Grain size analysis was performed on 110 sediment samples. Water discharge and velocity was measured on 274 points of the river cross section. A total of 182 bedload and suspended load sampling were also performed in 9 hydrometric gauging stations. The results show > 70% of the bedload and suspended load consist of sand and silt, respectively. Mineralogic composition of gravel, sand, and mud fraction of the sediments indicates a dominant volcanic origin. Two gravelly facies (Gmm, Gcm), five sand facies (Sm, Sh, St, Sp, Sl), and one mud facies (Fm) were recognized. These facies have been formed by six architectural elements: channel (CH), gravity flow deposits (SG), gravel bars and bed forms (GB), sand bed form (SB), laminated sand sheet (LS), and fine grain overbank deposits (FF). Accordingly, the system is influenced by flood and the depositional model for its upstream is proposed as gravel-bar braided with sediment-gravity flow. The river in midstream and downstream shows characters of shallow gravel-bed braided and gravelly-sandy meandering rivers, respectively. Coarse-grained sediments feeding by the channel tributaries and alluvial fans have produced distinct discontinuities in the downstream fining trend.

Tectonostratigraphy-volcanic of Gajah-Ijo-Menoreh Tertiary volcanic formations in Kulon Progo mountain area, Yogyakarta-Indonesia

Kulon Progo Mountains areas are formed by three Tertiary volcanic bodies that formed during Oligocene-Miocene times. Each volcano shows different tectonic control and stratigraphic result. This research conducted through interpretations of volcanic bodies, construction of stratigraphic columns, construction of geological cross-sections, supported by lithological-structural data from the field and some previous researchs. There are four volcanism periods in Kulon Progo Mountains, namely Late Eocene Volcanism, Early Oligocene Volcanism/Gajah, Late Oligocene Volcanism/Ijo and Late Miocene Volcanism/Menoreh. Gajah shape shows there are collaps volcano at the southern part and it was happened before Ijo emerge. Volcanism of the Eocene-Gajah-Ijo period is related to the Palaeogene extensional rifting period. Ijo volcano covers the southern part of Gajah Volcano. Menoreh volcano emerges at Late Miocene and covers Gajah volcano and Jonggrangan Formation. Menoreh Volcanism is related to the period of Neogene Compressional Wrench that produces a sinistral fault with left-stepping left lateral fault pattern.

Middle-Late Eocene marine record of the Biga Peninsula, NW Anatolia, Turkey

The Eocene shallow marine deposits marking the first marine incursion in the Biga Peninsula (NW Turkey) after the collision of the Sakarya and Anatolide-Tauride plates were investigated based on paleontological, litho- and chrono-stratigraphic data. Larger Benthic Foraminifera (LBF) from patchily distributed outcrops were studied in order to i) revise the stratigraphy of Eocene shallow marine units, and ii) establish a modern biostratigraphic setting and a correlation scheme. The Sevketiye Formation (Fm.) is herein defined as a predominantly shallow marine clastic deposit with subordinate carbonates overlying the Camlica metamorphic rocks, and passing laterally to the Sogucak Fm., a carbonate unit that is widely represented in the Thrace Basin. The record of alveolinids, primitive developmental stages of heterosteginids, and orthophragminids in the Sevketiye Fm. suggests that this formation is part of the Shallow Benthic Zones (SBZ) 16?/17 (Late Lutetian?/Early Bartonian), SBZ17?/18 and SBZ19A (Early Bartonian/earliest Priabonian). The Sogucak Fm., which overlies the Eocene volcanics, on the other hand, yielded advanced developmental stages of heterosteginids, Spiroclypeus sp. and Nummulites fabianii lineages, implying a younger marine incursion during the Late Eocene (earliest Priabonian; SBZ19A). A drastic shift in the depositional regime is marked amid Priabonian by the deposition of deep-marine clastics and volcanoclastics of the Ceylan Fm. In conclusion, the Eocene Sea transgressed first Gokceada (in the Aegean Sea) during the Late Lutetian, then reached the Biga and Gelibolu peninsulas in the Bartonian, and finally led to the widespread deposition of carbonate and siliciclastic rocks in the Biga Peninsula and the Thrace Basin during the Late Bartonian and Priabonian.

40Ar-39Ar geochronology and petrogenesis of postcollisional trachytic volcanism along the İzmir-Ankara-Erzincan Suture Zone (NE, Turkey)*

The obliteration of the Neo-Tethyan Ocean and collision of the microplates along the northern part of Turkey led to the development of the Izmir-Ankara-Erzincan suture zone (IAESZ). After the collision of Pontides with the Central-Anatolian Crystalline Complex (CACC) in the Paleocene, a new phase of extension and volcanism concomitantly developed along the northern (Almus; Pontides) and southern (Yildizeli; CACC) sides and along the IAESZ during the Middle Eocene time interval. The first products of the Middle Eocene volcanism in these areas are represented by calc-alkaline to alkaline (basic-intermediate) volcanic and volcanoclastic units together with late-stage trachytic dikes, plugs, and stocks. The mantle source area of both volcanic units displays a metasomatized character, which was dominantly fluxed by sediment-sourced melts. The partial melting of the metasomatized source area gave rise to first-stage basic-intermediate volcanism in the crustal levels. Simultaneously with the generation of the first-stage volcanism, basaltic trachyandesitic shallow-seated magma mushes were also developed. The reactivation of these shallow-seated mushes by late-stage extensional tectonics gave rise to the development of trachytic volcanism in both regions, which have a high-K to shoshonitic character. Almus trachytic lavas are phenocryst-poor and have differentiated Mg# numbers (avg. 26). On the other hand, Yildizeli trachytic lavas have a broad compositional range (benmoreite to latite); they are phenocryst-rich and show more basic character (Mg# avg. 40). Trachytic volcanism in both areas is largely controlled by fractional crystallization of similar basaltic trachyandesitic parental magma with minor assimilation of the upper crustal lithologies. 40Ar-39Ar ages from sanidine phenocrysts from both areas also confirm that trachytic volcanism in both regions developed nearly coevally in different tectonic blocks (~41?40 Ma). Generation of similar volcanism on the different tectonic blocks during the postcollisional stage was probably governed by a regional-scale delamination and/or lithospheric removal-related tectonomagmatic processes.

Late Eocene volcanism in North Patagonia (42°30′–43°S): Arc resumption after a stage of within-plate magmatism

Mid-Cenozoic widespread arc magmatism in North Patagonia extends from the forearc to the retroarc zones, representing an anomalous large volume when compared to the present-day arc zone and even other past arc configurations. It represents a crucial stage in Andean arc evolution as was developed after a period of arc waning and within plate magmatism. Controversies exist regarding the origin of these volcanic sequences, with scarce integrated field, geochemical and geochronological analyses. We focused our study on the El Maitén Belt, located in the present-day retroarc zone, particularly on a poorly studied section corresponding to the southern outcrops of this volcanic belt. This volcanism consists of basaltic and andesitic lava flows and interbedded pyroclastic deposits, whose emplacement was controlled by extensional tectonics as indicated by the occurrence of wedge-like strata associated with normal faults. A U-Pb age on the basal part of this section shows that magmatic activity started by 37Ma, earlier than previous studies that considered this volcanism as Oligocene. Geochemically, these rocks are part of the subalkaline and particularly tholeiitic series. All samples show trace element enrichments, depletions and ratios characteristic of arc magmas, though fluids and sediment imprint seem limited. On these bases, we propose decompression melting as the main process associated with the genesis of this volcanism. Therefore, this magmatic association constrained to the late Eocene represents the earliest evidence of arc volcanism in the Patagonian Andes, under an extensional regime, after a Paleogene waning of arc activity.